1. The Field of the Invention
The present invention relates to a hetero-branched radial polystyrene-polyisoprene (SIS) block copolymer used as a base polymer of pressure-sensitive adhesive compositions, and a preparation method thereof.
2. Related Prior Art
Different block polymers comprised of polystyrene-polyisoprene blocks have been used as a base polymer of pressure-sensitive adhesive compositions. Moreover, radial polystyrene-polyisoprene block polymers are known to be excellent in initial adhesion, holding power, workability, and heat stability.
Various coupling agents are available in use for the preparation of the radial polystyrene-polyisoprene block copolymers. Among the tetravalent coupling agents, SiCl4 is most preferred in the aspect of reactivity, bond stability, price, and supply stability.
Techniques for the preparation of a radial polystyrene-polyisoprene block copolymer using SiCl4 as a coupling agent are disclosed in, for example, U.S. Pat. Nos. 5,668,208; 5,552,493; and 6,534,593 B1.
As stated in the documents, an active lithium polymer having an isoprene terminal participates in a coupling reaction with SiCl4 to form a polymer having three branches due to steric hindrance. The use of butadiene to solve the problem that the steric hindrance prohibits 4-branch coupling is disclosed in U.S. Pat. No. 3,840,616. The use of butadiene for 4-branch coupling in the case of polymers having an isoprene or styrene terminal is described in a Fetters and Hadjichristidis et al., (Macromolecules, 7, 552, 1972 & 11, 668, 1978). The technologies applied to radial polystyrene-polyisoprene block copolymers based on the results of the researches are U.S. Pat. Nos. 5,292,819; and 5,399,627, WO 9220725 and WO 9514727.
More specifically, these techniques include adding less than 10% butadiene to the terminal of the isoprene block, and inducing a coupling with SiCl4 to form 4 butadiene block branches. However, the use of the radial block copolymer having 4 butadiene block branches as a base polymer of adhesive compositions may cause a deterioration of adhesive capacity as disclosed in U.S. Pat. No. 6,534,593 B1 it is may also deteriorate heat stability of the adhesive compositions in the case of polystyrene-polyisoprene block copolymers comprised of a polystyrene-polyisoprene block alone, heat stability may be deteriorated.
Some techniques for block copolymers having both the isoprene block and the butadiene block for improvement of heat stability are already disclosed, for example, in U.S. Pat. Nos. 5,532,319; and 5,583,182.
In summary, the polystyrene-polyisoprene block copolymer forms a 3-branched polymer instead of a 4-branched polymer by a coupling reaction with a tetravalent coupling agent due to steric hindrance of the isoprene block terminal. To solve this problem, the addition of a small amount of butadiene to the isoprene block terminal is suggested. But, the use of butadiene may deteriorate adhesive capacity, while without using butadiene results in poor heat stability. Accordingly, there is a need for a novel design of the base polymer that maintains adequate adhesive capacity and viscosity stability.